The invention relates to a measuring equipment and a method for mapping the geology in an underground formation and which comprises at least one transmitter circuit with at least one transmitter coil and a current source, at least one switch inserted between the current source and the at least one transmitter coil and which during operation serves to connect the current source of the at least one transmitter circuit with the at least one transmitter coil to generate an electrical current for building up a magnetic field in the formation and to break the electrical current again to thereby cause the built-up magnetic field in the formation to decay, and at least one receiving circuit with at least one receiving coil to measure the decay signal.
Above mentioned measuring equipment and method, which among people skilled in the art is called transient electromagnetic sounding, have long been known and have widespread use in mineral prospecting and mapping of ground-water resources.
It is also known that portable equipment is used for mapping of ground-water resources. The equipment consists of a transmitter unit with belonging transmitter cable and a receiving unit with belonging receiving coil. On the surface of the ground, the transmitter cable is laid out in a square which forms the transmitter coil with a typical area of 40×40 meters and which is connected to the transmitter unit. The receiving coil is placed in the middle of the transmitter circle (Eng. central loop) or outside the transmitter circle (Eng. offset loop). Certain equipment also uses the transmitter coil as receiving coil (Eng. coincident loop).
Such portable equipment is known from international patent application WO 96/33426, which is disclosed herein as a reference to such known equipment. This portable equipment works by an electrical current being supplied to the transmitter coil from the transmitter unit, which typically has a value of 1–5 amperes. The current is abruptly cut off after a short period of time, typically 5–10 milliseconds, after which the magnetic field built up by the electrical current in the transmitter coil decays. In the receiving coil a voltage is thereby induced which is representative of the decay.
The measuring in the field is typically done along profile lines in such a way that the separate transient electromagnetic soundings are performed with predetermined mutual distances, typically 200–500 meters. Thereby, an estimate of the electrical resistance structure of the formation and thus of the geological formation along the profile lines can be achieved by interpretation of the separate soundings. The interpretation results from the separate line profiles can be grouped through interpolation to a surface covering map or sectional profile of the electrical resistance relations and thus through interpretation of the results to a model of the geological formation in an area.
The transient electromagnetic method is used with great success in mineral exploration since many mineral formations are highly electrically conductive with respect to the mother rock in which they are deposited. Since large areas need to be investigated, instruments for use from fix-wing airplanes have been developed. Thus, measurements in close profile lines can be made and the results can be grouped to surface covering maps over great areas. The decay signal can however not be measured at the early decaying times since it is necessary to use relatively large transmitting currents and airspeeds and for the airplane to fly at an altitude corresponding necessarily to the assignment.
During recent years the transient electromagnetic method has gained a strong footing in mapping of ground-water resources. In order to be able to perform this mapping satisfactorily, it is necessary to use instruments which can measure the decay signal at decay times from a few microseconds to 5–10 milliseconds. Contrary to the instrument used in aviation measurement, the decay signal can be measured in the time interval from a few microseconds after the transmitter current has been cut off to 5–10 milliseconds with instruments for use on the ground surface. As mentioned instruments with high transmission currents are used for aviation measurements, which means that the current in the transmitter coil will decay slowly after the current supply has been cut off in form of a so called cut-off current. Thus the measuring of the decay signal at early decay times will be influenced by the cut-off current in the transmitter coil and be unusable for use in an interpretation. Similarly, the size of the decay signal highly depends on the distance between the transmitter and receiving coils and their distances to the ground surface. Finally, the transmitter coil encircles the plane and thereby induces eddy currents herein. The receiving coil registers the thereby formed magnetic field.
It is very difficult to compensate or correct precisely enough for these influences. During mineral exploration, these influences are without importance since the measured decay signal has an accuracy that can provide the necessary indications of the presence of conductive mineral deposits. The instruments can however not be used for mapping of ground-water resources since in these cases, it is necessary to acquire a considerably more detailed and accurate knowledge of the decay signal at both late and early decay times.
It has therefore been the common conception that transient electromagnetic sounding for use in mapping of ground-water resources had to be performed with portable equipment for use on the ground surface. In mapping of ground-water resources, it is however often large areas that have to be covered in order to obtain the regional understanding and insight of the ground-water aquifer connections and structures. This means that at such investigations, a large number of soundings have to be performed in order to be able to construct a map of the geological structures.
The work herewith is extensive and is not carried out in most cases because of the associated financial costs, and this in spite of the fact that experience has shown that the transient electromagnetic method can give a detailed picture of the geological structure of the underground. A picture that is necessary in Denmark and other countries is necessary to be able to make optimal planning of the use of the gradually more sparse water resources. Because of crops in the fields and soft ground especially in the spring and autumn seasons there are furthermore long periods of time when it is not possible to conduct sounding in the fields.
There is therefore a great need for a compact measuring equipment which can perform the same measurements with the same data quality of the late and early decay times, which until now have been carried out on the ground surface, while at the same time being capable of being transported in the air by e.g. a helicopter. In international patent application WO 96/33426, fundamental transient- sounding is further described and how an improvement of the measurement results can be obtained by transmitting magnetic moments in chronological order. Even though this method is effective and reliable, it is not optimal for e.g. airborne transient sounding where the mechanical construction, such as size and reduction of coupling between transmitter and receiver, is very important. The present invention solves these problems and thereby provides a considerably improved system which is well suited for airborne transient sounding.
In U.S. Pat. No. 3,835,371 a technique is described for measuring the presence of electrically conductive objects on the seabed by a transmitter coil which is dragged through the water. When the transmitter coil is dragged above an electrically conductive object in the water, eddy currents will be induced in the object. These eddy currents form a secondary field which is measured by a receiving coil located in the immediate vicinity of the transmitter coil. By placing the transmitter coil and the receiving coil perpendicular to each other, the flux from the transmitter coil through the receiving coil is minimized, and even weak secondary fields can thereby be measured without interference from the transmitted magnetic field of the transmitter coil.
Since it is the response of the soil and its characteristics that are interesting at transient sounding, the receiving coil must necessarily have the same planar orientation as the transmitter coil. The technique known from the above U.S. Pat. No. 3,835,371, which operates with coils that are perpendicular to each other, is close to useless for transient sounding which works with parallel coils.
U.S. Pat. No. 5,557,206 describes another system for transient sounding. In this case, two concentric coils, one having a larger diameter than the other, are used to measure responses from the geological formation. The two coils are serially connected in such a way that they obtain opposite current directions. The field strength in the middle of the coil can be minimized or reduced to zero by adjusting the coils number of turns and area.
The strength of the coils' far field is however not minimal or zero since this strength depends on the area of the coils multiplied by number of turns of each coil and the current. The far field can be used for measuring a geological deposit by induction of a field which can be measured in a receiving coil placed in the middle of the transmitter coils. This solution cannot be used, however, for measurement of earth responses from transient fields which are obtained by a quick cut-off of the current.
When the current in the two transmitter coils, which is used in measuring method known from U.S. Pat. No. 5,557,206, is cut off, the cut-off time depends on the uneven self-induction and self capacity of the coils. The cut-off time in the two coils is thus not identical, resulting in the generated magnetic fields being unable to even out each other in the middle of the coils. Measurement of the response from the ground structure will thus be distorted by the interference from the cut-off current in the transmitter coils.
In US patent application no. US 2003/0016131 A1 a measuring arrangement is described which operates with a transmitter coil with many closely placed wires. By building the transmitter coil in this way, a uniform induction field is obtained, which provides a uniform induction in objects in the ground. The response from these objects can thus easily be recognized. The purpose with this arrangement is to detect defined objects such as cables, pipes and unexploded mines in the ground and not geological deposits. Noise from other sources such as the earth response is eliminated by using two or more receiving coils.
Accordingly, there is a need for improved devices and these are now provided by the present invention.